Telephone answering machines employing tape recorders for playing an outgoing "hello" message (i.e., outgoing announcement or OGA), and for recording incoming messages, are now commonly available. In operation, when the telephone answering machine answers an incoming call, it plays the "hello" message, emits a "beep" tone, and records an incoming message onto the tape. It sometimes happens that the called-party is at home and desires to answer the telephone call himself rather than let the telephone answering machine answer the call. In such a case, it is desirable that the outgoing announcement be automatically terminated in response to the lifting of the receiver of an extension telephone, rather than forcing the user to wait until the outgoing announcement finishes. Such a system requires an automatic disconnect feature including a detector circuit to detect the "off-hook" condition of the extension telephone. A telephone answering machine having an automatic disconnect feature, and including an "off-hook" detector is known from the GE telephone answering machine model number 9882 manufactured by Thomson Consumer Electronics, Inc. Indianapolis, Ind. The answering machine model number 9882 was manufactured and sold to the public on or about May 1, 1989, and a manual describing the unit was published at the same time, on or about 5/1/89.
In these, and in similar, telephone answering machines, the detector can be "fooled" into terminating the outgoing announcement and disabling the recording feature by misinterpreting other signal conditions on the telephone line as extension "off-hook" conditions. Some of the falsely interpreted signal conditions are: a pulse indicative of "call-waiting", momentary loop instability during the first few seconds of the call, and call termination by the calling party (i.e., if the calling party hangs-up, the time and date message may not be added to the tape).
The extension telephone off-hook detector must be made very sensitive in order to detect the small changes in telephone line voltage which are indicative of an off-hook extension telephone. Unfortunately, the three often-misinterpreted telephone signal conditions (i.e., loop interruptions) set forth above cause relatively large changes in telephone line voltage. There has clearly been a long-felt need for a reliable solution to the problem of detecting the small signal change caused by the off-hook extension telephone, while not detecting the large loop voltage changes caused by the above-noted telephone line signal conditions. The above-noted GE telephone answering machine model number 9882 includes a manually-operable switch for disabling the automatic disconnect feature in the event that false detection of telephone line signal conditions occurs frequently enough to become an annoyance.